Windows of the type involved are generally well known, as a rule they are mounted in the front doors of baking ovens or the like. The following description is of household baking ovens, although the invention is also useful for any other device that has a window which separates an inner chamber with elevated temperature from the outer environment and through which a view from the outside into this chamber with elevated temperature is possible.
In contrast to the other walls surrounding an inner chamber with elevated temperature, in the case of windows it is not possible to apply an effective heat insulation, for example on the basis of mineral wool or the like. Therefore it is just these windows that have the problem of heating up too much on the outside. In order to decrease the external temperature of a window, in general multiple-pane arrangements are already in use, in which the inner side of the outer pane no longer contacts the chamber with elevated temperature directly, so that heating of the outer pane by direct heat transfer is already prevented. Still, with several panes at a distance from each other with air spaces separating them, the outside pane heats up by radiation from the inner chamber with elevated temperature, whereby this radiation affects, in part, the inner panes and is absorbed directly by the outside pane, but in part is also absorbed by the inner panes which then in turn transmit heat radiation to the next pane via the air-filled intermediate space by heat conduction and convection.
There are also versions of windows known in which an external pane is held at a distance from the other panes in such a way that the frame of the arrangement is open at least on the top and the bottom between the outside pane and the next inward pane so that between this outside pane placed in front and the next inner pane air convection can take place in the ambient chamber outside the device whereby for one thing the air that heats up between the two outer panes is exchanged by circulation of the ambient air and for another thing the outer pane placed in front is continuously cooled by this air stream.
There is also the possibility of using thermochrome panes inside, the radiation permeability of which is reduced as the temperature increases. However, this impairs the view through the window.
Providing one or more of the panes of a multi-pane arrangement with a coating is also known, which either reflects infrared radiation or decreases the emission of infrared radiation. A coating that exhibits one of these two properties is a coating of tin dioxide, for example. If it is applied on the inner side of a pane that is turned toward the chamber with elevated temperature, it reflects a part of the infrared radiation back into the inner chamber, it decreases the infrared radiation to the outside to the next outer pane. The panes can also be coated on both sides.
In spite of the provisions explained above, it is not unusual in household baking ovens that are operated at temperatures of 400.degree. C. to 450.degree. C. and above, especially with self-cleaning by pyrolysis, that the outer panes of a window are elevated to a temperature of up to about 150.degree. C. In order to ensure safety when the window is touched by an operator, there is an effort in the relevant standards to limit the maximum permissible external temperature of such windows to 80.degree. C.